Redhat PXE boot problems

Hi:

I'm hoping someone can help with my PXE boot issues. I'm using Redhat
AS4 on Dell 6850s and 2950s - both servers act the same.

I'm trying to set up a kickstart system. I've got DHCP running on
Solaris 9 (Internet Systems Consortium DHCP Server V3.0.2). On boot,
the Dell box gets an IP address including the next-server and boot file
name (seen using tethereal), but that is it - no attempt to reach the
tftp server is made.

If I put DHCP and TFTP on another Redhat box and hook up with a
crossover cable, everything works splendidly. If I hook things up over
a hub or even isolate everything through it's own switch, everything
works fine. If I then take that Redhat box and put it on the network
(going through Foundry stuff), I'm back to not trying to hit the TFTP
server.

I would be more than happy to blame it on DHCP on Sun, but I get the
same results from Redhat over a network (with portfast turned on the
the port the Dell box is plugged into), and with a configuration that
works when directly connected.

I'd be happy to blame it on the network, but - when I snoop the traffic
over the crossover cable, I see:
dellserver -> bcast dhcp discover
dhcpserver -> bcast dhcp offer (including next-server and boot
filename)
dellserver -> bcast dhcp request
dhcpserver -> bcast dhcp ack
dellserver -> bcast arp request for next-server

the arp request gets a response, and the TFTP traffic follows. When I
plug everything into the real network (no crossovers), and snoop the
line on the same segment as the dell box, I see the dhcp traffic but
the dell box never does an arp request for the next server.

I can't blame it on the network until I know why the dell box isn't
broadcasting an arp for the TFTP server, as it does when directly
connected - because obviously if the dell box never asks for the
address, it will never get it and thus won't be able to continue.

Here is my Sun dhcpd.conf. This exact configuration works on a Dell
directly connected - but not switched over the network.
server-identifier host.domain.com;
authoritative;
key mykey {
algorithm hmac-md5;
secret "XXXX";
};
default-lease-time 86400;
max-lease-time 604800;
option subnet-mask 255.255.0.0;
option broadcast-address 10.1.255.255;
option routers 10.1.1.1;
option domain-name-servers 10.1.5.98,10.1.1.2,10.1.1.17;
option domain-name "domain.com";
ddns-update-style interim;

allow bootp;
allow booting;

group {
next-server 10.1.2.70;
filename "pxelinux.0";

host dellhost.domain.com {
hardware ethernet 00:13:72:41:64:31;
fixed-address 10.1.7.111;
option host-name "dellhost";
}
}

subnet 10.1.0.0 netmask 255.255.0.0 {
range 10.1.150.10 10.1.150.100;
}

What am I missing? Why isn't the Dell box sending an ARP for the TFTP
server?

Thanks all for your help...

Bill